3.a …and install them
Both files are compressed Debian packages which can be installed as usual, at least on 32-bit systems. People using this architecture can continue with step 4.

3.b Special attention for 64-bit
64-bit systems have to force the installation:

sudo dpkg -i --force-architecture ./cnijfilter-*
sudo dpkg -i --force-architecture ./scangearmp-*
Having installed it that way, we have to check whether the needed libraries are also available in a 32-bit version. Therefor there is the tool getlibs. In fact we have to go through all binaries installed by the Canon software, run them in getlibs and control if the corresponding libraries are installed. By doing that on my Lucid system, I found out that only /usr/bin/scangearmp has to be checked.

sudo getlibs /usr/bin/scangearmp
All missing libs should be listed that way and it prompts to download them. In my situation that was only the case for libgimp2.0.

4. Searching the printer
If the printer is not listed in the printer dialog by now, you can add it with the common printer tools of your desktop environment. Gnome, for instance, has the printer GUI in its System/Administration menu.

5. Editing the printer profile
This step is optional. Your printer should be able to print now. Expecting more features from your printer, you can add special functionality like black-white-printing or the like in your printer profile.
I recommend to edit the following files lest an update overwrites your changes (names may vary, backups of original files self-evident).

/usr/share/ppd/canonmp640.ppd
/etc­/cups/ppd/Canon-MP640.ppd
By editing these files with root access, you can

6. Final test
The printer driver comes with a handy status tool which is also the place to go if you want to find out what your printer currently does or if you want to check the availability. So cngpijmonmp640 shows you the printer status and ink level (screenshot at the top).

By the way…
Not only does the printer work, but of course also the scanner. I tested it successfully with xsane, ScanGearMP (single and over GIMP) and SimpleScan. Keep in mind that the sane-backends included in Lucid are too old for the MP640—but with the PPA mentioned above it’s very easy to have full compatibility by simply updating the package.

Moreover there is a RPM package for other distributions and the source.